Barrier sealing system

ABSTRACT

A barrier seal system comprises a rotatable shaft disposed within an outer housing. The rotatable shaft axially extends from a high-pressure region to a low-pressure region within the outer housing. At least a portion of the rotatable shaft, typically the portion disposed adjacent to the low-pressure region, comprises an oleophobic surface. The system further comprises a barrier seal that radially extends from the outer housing to the rotatable shaft and defines a gap between the barrier seal and the rotatable shaft. A barrier gas injector is provided for injecting a barrier gas into the gap to flow from the high-pressure region to the low-pressure region.

BACKGROUND

The invention relates generally to barrier sealing technologies and morespecifically to methods and apparatus for low-flow oil barrier sealingtechnologies with improved surface properties.

Many current applications, including for example rotating machinery, usea barrier seal to separate lubricant from the internals of a respectivemachine to prevent damage and degradation of the various internalmachine parts. Barrier seals, or tertiary seals, are often used inconjunction with dry gas seals in many applications, including forexample, compressor technology like gas centrifugal compressors. Evergreater demands are being placed on dry gas seals and their supportsystems, requiring continual improvements in the design of the dry gasseal environment, both internal and external to the compressor.Contamination is the leading cause of dry gas seal operationaldegradation and reduced reliability.

A barrier seal is typically required on the outboard side of a dry gasseal, between the gas seal and the compressor bearing housing area. Thisseal is typically buffered with air or nitrogen. The primary function ofthe barrier seal, in this application, is to prohibit the flow ofbearing lubrication oil or oil mist into the dry gas seal. Contaminationof the dry gas seal from the lubrication oil can occur when the barrierseal fails to function as intended. Even with the application of a purgegas to prevent the migration of the lubrication oil through the barrierseal, the lubrication oil frequently wicks or leaks along the shaft intothe internals of the machine and to the dry gas seal. If the purge orbarrier gas is applied at a sufficient flow rate, for example, greaterthan about 16 feet per second over the life of the seal, the velocityboundary layer that forms is typically thin enough to preclude oilmigration no matter how thin the lubricant film becomes. It isimportant, however, to minimize the quantities of the purge or barriergas required to preclude the oil migration.

Accordingly, there is a need in the art for an improved barrier sealthat can prevent the migration of lubricants into the internals of amachine while minimizing the quantities of separation gas required to doso.

BRIEF DESCRIPTION

A barrier seal system comprises a rotatable shaft disposed within anouter housing. The rotatable shaft axially extends from a high-pressureregion to a low-pressure region within the outer housing. At least aportion of the rotatable shaft, typically the portion disposed adjacentto the low-pressure region, comprises an oleophobic surface. The systemfurther comprises a barrier seal that radially extends from the outerhousing to the rotatable shaft and defines a gap between the barrierseal and the rotatable shaft. A barrier gas injector is provided forinjecting a barrier gas into the gap to flow from the high-pressureregion to the low-pressure region.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of one embodiment of the instantinvention.

FIG. 2 is a schematic illustration of another embodiment of the instantinvention.

FIG. 3 is a schematic illustration of another embodiment of the instantinvention.

FIG. 4 is a schematic illustration of an aspect of another embodiment ofthe instant invention.

FIG. 5 shows photographs of an oil droplet on silicon posts withdifferent b/a ratios.

FIG. 6 is roll-off droplet radius as a function of b/a for silicon poststructures.

FIG. 7 is a plot of post sizes for impact resistant textures as afunction of b/a.

FIG. 8 is a side-view pictorial representation of drag force on adroplet.

FIG. 9 is a top-view pictorial representation of drag force on adroplet.

FIG. 10 depicts a textured surface comprised of an array of squareposts.

FIGS. 11-14 are plots demonstrating drag force reduction on a variety ofsurfaces.

DETAILED DESCRIPTION

A barrier seal system 10 comprises a rotatable shaft 12 (with or withouta shaft sleeve (not shown)) disposed within an outer housing 14 and abarrier seal 16 that radially extends from the outer housing 14 to therotatable shaft 12. The rotatable shaft 12 axially extends from ahigh-pressure region 18 to a low-pressure region 20. The barrier seal 16and the rotatable shaft combine to define a gap 22 therebetween. Thesystem 10 further comprises a barrier gas injector 24 for injecting abarrier gas 26 into the gap 22 to prevent oil migration from thelow-pressure region 20 to the high-pressure region 18.

Conventionally, the barrier gas injector 24 injects the barrier gas 26,for example air or an inert gas like nitrogen, at a sufficient flowrate, for example, greater than 16 feet per second over the life of theseal. At this flow rate, the velocity boundary layer is typically thinenough to preclude oil migration no matter how thin the lubricant filmbecomes. It is important, however, to minimize the quantities of thepurge gas required to preclude the oil migration. Accordingly, in oneembodiment of the instant invention, at least a portion of the rotatableshaft 12 (or shaft sleeve), typically adjacent to the low-pressureregion, comprises an oleophobic surface 28.

As used herein, the term “oleophobic surface” means any surface thatreduces the tendency for an oil to attach to that surface or form a filmon that surface, including all superoleophobic surfaces. Oleophobicsurfaces are characterized by reduced build-up and more facile removalof oils from the surface, compared to surfaces that are not oleophobicin nature.

Accordingly, because a portion of the rotatable shaft 12 adjacent thelow-pressure region 20 comprises oleophobic surface 28, the quantitiesof the injected barrier gas 26 required to preclude the oil migrationare greatly reduced because any oil that contacts that portion ofrotatable shaft or sleeve will bead up or have steep wetting angles andthe barrier gas will be able to push the oil away from the barrier sealeasier. By reducing the quantities and flow rate of the required barriergas 26, the barrier seal system 10 is greatly improved over aconventional system, making the system 10 more resistant to oil leakingor wicking and reducing the costs associated with the system'soperation.

The oleophobic surface 28 on the rotatable shaft 12 increases thewetting angle of the oil contacting the shaft such that the oil cannotmaintain a film thickness that is small enough to stay inside a lowvelocity boundary layer of the barrier gas 26. Therefore, the highervelocity in the bulk stream of the barrier gas 26 can force the oil awayfrom the barrier seal 16 easier and preclude oil migration and reducethe quantities of barrier gas 26 required to do so.

As shown in FIG. 2, the barrier seal system can include additional typesof sealing arrangements, for example brush seals, labyrinth seals,carbon face seals and combinations thereof. In FIG. 2, barrier sealsystem 50 includes multiple labyrinth seals 52 and multiple brush seals54. In this embodiment, a centrally located barrier gas injector 56 isused to inject the barrier gas into the gap 58.

As used herein, the “contact angle” or “static contact angle” is theangle formed between a stationary drop of a reference liquid and ahorizontal surface upon which the droplet is disposed, as measured atthe liquid/substrate interface. Contact angle is used as a measure ofthe wettability of the surface. If the liquid spreads completely on thesurface and forms a film, the contact angle is 0 degrees. As the contactangle increases, the wettability decreases.

Referring to the drawings in general and to FIG. 3 in particular, itwill be understood that the illustrations are for the purpose ofdescribing a particular embodiment of the invention, and are notintended to limit the invention thereto. FIG. 3 is a schematiccross-sectional view of a surface of the rotatable shaft 12 coated withan oleophobic surface 28, according to one embodiment of the invention.Article 100 comprises a surface 102. As used herein, the term “surface”refers to that portion of the article 100 that is in direct contact withan ambient environment surrounding the article 100. The surface mayinclude the substrate, the features, or the surface modification layerdisposed over the substrate, depending on the specific configuration ofthe article. Surface 102 has low oil wettability. One commonly acceptedmeasure of the oil wettability of a surface 102 is the value of thestatic contact angle 104 formed between surface 102, and a tangent 106to a surface of a droplet 108 of a reference oil at the point of contactbetween surface 102 and droplet 108. High values of contact angle 104indicate a low wettability for the reference oil on surface 102.

As used herein, “oil” is to be understood as having its common meaningto cover a wide variety of unctuous substances not miscible with water.Examples include oils of animal, vegetable, or mineral origin, as wellas synthetic oils. Particular examples of oils include petroleum-basedproducts, such as crude oil and products distilled therefrom, such askerosene, gasoline, paraffin, and the like. In some embodiments, the oilcomprises an industrial lubricant such as bearing oil or light turbineoil. In one embodiment of the invention, the oleophobic surface 28 (FIG.1, FIG. 2) is a surface that generates a static contact angle with oilof at least about 30 degrees.

In one embodiment, a surface 150 comprises a material having a nominalliquid wettability sufficient to generate, with reference to an oil, anominal contact angle of at least about 30 degrees, as shown in FIG. 4.For the purposes of understanding the invention, a “nominal contactangle” 152 means the static contact angle 152 measured where a drop of areference oil 154 is disposed on a flat, smooth (<1 nm surfaceroughness) surface 150 consisting essentially of the material. Thisnominal contact angle 152 is a measurement of the “nominal wettability”of the material. In one embodiment, the nominal contact angle, withreference to an oil, is at least about 50 degrees. In one embodiment,the nominal contact angle, with reference to an oil, is at least about70 degrees. In one embodiment, the nominal contact angle, with referenceto an oil, is at least about 100 degrees. In yet another embodiment, thenominal contact angle, with reference to an oil, is at least about 120degrees.

Surface 28 (FIGS. 1 & 2) and surface 102 (FIG. 3) comprise at least onematerial selected from the group consisting of a ceramic, anintermetallic, and a polymer. Suitable ceramic materials includeinorganic oxides, carbides, nitrides, borides, and combinations thereof.Non-limiting examples of such ceramic materials include aluminumnitride, boron nitride, chromium nitride, silicon carbide, tin oxide,titania, titanium carbonitride, titanium nitride, titanium oxynitride,stibinite (SbS₂), zirconia, hafnia, and combinations thereof. In certainembodiments, the surface comprises an intermetallic. Examples ofsuitable intermetallic materials include, but are not limited to, nickelaluminide, titanium aluminide, and combinations thereof. Polymermaterials that may be used in surface 102 include, but are not limitedto polytetrafluoroethylene, fluoroacrylate, fluoroeurathane,fluorosilicone, modified carbonate, silicones and combinations thereof.The material is selected based on the desired contact angle and thefabrication technique used.

In another embodiment, surface 102 further comprises a texturecomprising a plurality of features 110 (FIG. 3). A surface 102,comprising a material of comparatively high nominal wettability, with aspecific texture, as described in detail below, has a significantlylower wettability than that inherent to the material from which thesurface is made. In particular, surface 102 has an effective wettability(that is, wettability of the textured surface) for the reference oilsufficient to generate an effective contact angle greater than thenominal contact angle. The effective contact angle depends, in part, onthe feature shape, dimensions, and spacings, as will be described indetail below.

As described above, in one embodiment, surface 102 has a texturecomprising a plurality of features 110. The plurality of features 110may be of any shape, including at least one of depressions, protrusions,nanoporous solids, indentations, or the like. The features may includebumps, cones, rods, wires, channels, substantially spherical features,substantially cylindrical features, pyramidal features, prismaticstructures, combinations thereof, and the like. Numerous varieties offeature shapes are suitable for use as features 106. In someembodiments, as shown in FIG. 1, at least a subset of the plurality offeatures 110 protrude above the surface 102 of the article. In someembodiments at least a subset of the plurality of features 110 is aplurality of cavities 112 disposed in the surface 102. In someembodiments, at least a subset of the features 110 has a shape selectedfrom the group consisting of a cube, a rectangular prism, a cone, acylinder, a pyramid, a trapezoidal prism, and a hemisphere or otherspherical portion. These shapes are suitable whether the feature is aprotrusion 110 or a cavity 112.

The size of features 110 (FIG. 3) can be characterized in a number ofways. Features 110 comprise a height dimension (h) 114, which representsthe height of protruding features above the surface 102 or, in the caseof cavities 112, the depth to which the cavities extend into the surface102. Features 110 further comprise a width dimension (a) 116. Theprecise nature of the width dimension will depend on the shape of thefeature, but is defined to be the width of the feature at the pointwhere the feature would naturally contact a drop of liquid placed on thesurface of the article. The height and width parameters of features 110have a significant effect on wetting behavior observed on surface 102.

Feature orientation is a further design consideration in the engineeringof surface wettability in accordance with embodiments of the presentinvention. One significant aspect of feature orientation is the spacingof features. Referring to FIG. 3, in some embodiments features 110 aredisposed in a spaced-apart relationship characterized by a spacingdimension (b) 118. Spacing dimension 118 is defined as the distancebetween the edges of two nearest-neighbor features. Other aspects oforientation may also be considered, such as, for instance, the extent towhich top (or bottom for a cavity) deviates from being parallel withsurface 102, or the extent to which features 110 deviate from aperpendicular orientation with respect to the surface 102.

The plurality of features 110 (FIG. 3) making up texture 110 need not beconfined to the surface 102 or a region immediately proximate to thesurface 102. In some embodiments, article 100 further comprises a bulkportion 120 disposed beneath surface 102, and the plurality of features110 extend into bulk portion 120. Distributing features 110 throughoutthe article 100, including at the surface 102 and within the bulkportion 120, allows surface 102 to be regenerated as the top layer ofsurface erodes away.

In certain embodiments, the surface comprises a surface energymodification layer (not shown). In certain cases, the surface energymodification layer comprises a coating disposed over a substrate. Thesubstrate may comprise at least one of a metal, an alloy, a plastic, aceramic, or any combination thereof. The substrate may take the form ofa film, a sheet, or a bulk shape. The substrate may represent article100 in its final form, such as a finished part; a near-net shape; or apreform that will be later made into article 100. Surface 102 may be anintegral part of the substrate. For example, surface 102 may be formedby replicating a texture directly onto the substrate, or by embossingthe texture onto the substrate, or by any other such method known in theart of forming or imparting a predetermined surface texture onto asubstrate surface. Alternatively, surface 102 may comprise a layer thatis disposed or deposited onto the substrate by any number of techniquesthat are known in the art.

Example—Making silicon articles with oleophobic properties: Siliconsubstrates were provided via lithography with right rectangular prismfeatures about 3 micrometers in width (a) and having various b/a ratios.The substrates were then placed in a chamber with a vial of liquidfluorosilane, and the chamber was evacuated to allow the liquid toevaporate and condense from the gas phase onto the silicon substrate,thereby creating a film on the surface. The contact angle was recordedas a function of b/a ratio. FIG. 5 shows the photographs of oil droplets62 on silicon posts 60 with different b/a ratios. The figure listsstatic contact angle of oil (a light turbine oil in this case) ondifferent textures. The ease of roll-off was measured by determining theangle of tilt from the horizontal needed before a drop will roll off ofa surface. A drop that requires a near vertical tilt is highly pinned tothe surface, whereas a drop exhibiting easy roll-off will require verylittle tilt angle to roll off the surface. In some embodiments, the dropwill roll off of the surface at the point where the force of gravitypulling on the drop equals the force pinning the drop to the surface.This situation can be represented by the following expression:

ρVg sin α=2πμβr  (1);

where ρ is the liquid density, V is the volume of the drop, g is thegravity constant, α is the angle of inclination from the horizontal, μis the pinning parameter, β is the fraction of the contact line that ispinned, and r is the radius of the contact area of the drop with thesubstrate. μ, the pinning parameter, is a material constant that isindependent of the surface texture, but β and r are functions of thetexture. The texture, in some embodiments, is represented by theparameters a, b, and h of the features. Based on the oil roll-off onsmooth silicon with fluorosilane, the pinning parameter μ was calculatedto be 0.029 N/m. For water, the pinning parameter is of the order of0.013 N/m. Table 1 lists the contact angles for different b/a ratios.

TABLE 1 Contact angles for different b/a ratios. Contact angle (withSample a (micrometers) B/a reference to oil) 1 3 0.33 110 2 3 0.5 151 33 0.75 149 4 3 1 137 5 3 1.5 144 6 3 2 132 7 3 4 90 8 3 5 83 9 3 7.5 10310 3 10 81

Table 1 shows the effect of varying b/a on the contact angle. Thecontact angle measured on a control specimen having a smooth(non-textured) surface coated with fluorosilane was about 88 degrees.Above b/a of 4, the contact angle decreases as the drop settles into awetting state under its own weight.

As a practical matter, design considerations are applied to arrive at asurface design that promotes a high contact angle and easy droproll-off. FIGS. 6 and 7 show the results of work aimed at validating theabove analysis, and the plots illustrated in these figures may be usedto select suitable textures for a range of applications, for a givencombination of oil type and surface material. FIG. 6 shows the plot 200of maximum diameter of the drop required for the drop to roll off of atexture comprised of posts described above. FIG. 7 gives plot 300, themaximum diameter of the drop required for the drop to roll off on atexture comprised of pore structures.

FIG. 8 (side-view) and FIG. 9 (top-view) are pictorial representationsthat depict the drag force on a droplet. The drag force is calculatedusing the following equation:

$\frac{F_{d}}{\mu \; V^{1/3}} = {{\beta \left( {24\; \pi^{2}} \right)}^{1/3}\left( \frac{\sin \; \theta}{\left( {2 - {3\; \cos \; \theta} + {\cos^{3}\theta}} \right)^{1/3}} \right)}$

where Fd is the drag force, μ is the hysteresis coefficient, θ is thecontact angle, β is the texture coefficient, and V is the volume of thedroplet.

FIG. 10 depicts a textured surfaces comprised of an array of squareposts. The texture coefficient is represented by the following equation:

β=1/(1+b/a)

where b is the distance between adjacent posts and a is the width of arespective post. The contact angle on a textured surface is representedby the following equation:

cos(θ_(t))=−1+[(1+cos(θ_(S)))/(1+(b/a))²]

where θ_(t) is a contact angle on a smooth surface.

For an oil droplet sitting on a textured oleophobic material, θ will begreater than 90 degrees and β will be less than 1 (assuming the droplethasn't penetrated the texture). As a result, the drag force on the oildroplet will be significantly less for a textured oleophobic materialcompared to that on a smooth non-oleophobic surface. The drag force canbe reduced further by choosing a material with low hysteresiscoefficient, μ. This has been illustrated in FIG. 11-14. All the plotsare based on Exxon Teresstic GT 32 Turbine Oil interacting with aSilicon surface.

FIG. 11 is a graphical depiction of a non-dimensional drag force versusthe contact angle for a smooth surface. As one can see from FIG. 11, thedrag force can be substantially reduced by employing a smooth oleophobicsurface thereby increasing the oil droplet contact angle.

FIG. 12 is a graphical depiction of a non-dimensional drag force versusthe contact angle for textured surface. FIG. 12 illustrates that thedrag force reduction can be further enhanced by using a texturedoleophobic material. This example is based on Exxon Teresstic GT 32Turbine Oil on a textured Silicon surface coated with flurosilane. Thecontact angle of the Exxon Teresstic GT 32 Turbine Oil on a smoothsilicon surface is 83 degrees and the hysteresis coefficient is equal to0.029 N/m.

FIG. 13 is a graphical depiction of drag force versus contact angle fora 1 micro liter droplet on a smooth surface.

FIG. 14 is a graphical depiction of drag force versus contact angle fora 1 micro liter droplet on a textured surface. This example is based onExxon Teresstic GT 32 Turbine Oil on a textured Silicon surface coatedwith flurosilane. The contact angle of the Exxon Teresstic GT 32 TurbineOil on a smooth silicon surface is 83 degrees and the hysteresiscoefficient is equal to 0.029 N/m.

FIGS. 13 and 14 depict the actual drag force reduction with the changein oil contact angle and in the presence of texture.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A barrier seal system comprising: a rotatable shaft disposed withinan outer housing and axially extending from a high-pressure region to alow-pressure region, wherein at least a portion of said rotatable shaftdisposed adjacent said low-pressure region comprises an oleophobicsurface; a barrier seal that radially extends from said outer housing tosaid rotatable shaft and defines a gap between the barrier seal and therotatable shaft; a barrier gas injector for injecting a barrier gas intosaid gap to flow from said high-pressure region to said low-pressureregion.
 2. A barrier seal system in accordance with claim 1, whereinsaid barrier seal system is used within rotating machinery.
 3. A barrierseal system in accordance with claim 1, further comprising a lubricatingelement within the low-pressure region.
 4. A barrier seal system inaccordance with claim 3, wherein said barrier gas injector injects abarrier gas through said gap to prevent migration of said lubricatingelement from said low-pressure region to said high-pressure region.
 5. Abarrier seal system in accordance with claim 4, wherein said oleophobicsurface prevents film formation of said lubricating element adjacent tosaid gap.
 6. A barrier seal system in accordance with claim 1, whereinsaid oleophobic surface is an oleophobic coating.
 7. A barrier sealsystem in accordance with claim 1, wherein said oleophobic surface is anoleophobic texturing.
 8. A barrier seal system in accordance with claim3, wherein said lubricating element is oil.
 9. A barrier seal system inaccordance with claim 1, wherein said barrier seal is selected from thegroup consisting of brush seals, labyrinth seals and carbon face seals.10. A barrier seal system in accordance with claim 1, wherein saidbarrier seal is selected from the group consisting of contacting sealsand non-contacting seals.
 11. A barrier seal system in accordance withclaim 1, wherein said barrier gas is injected at a mass flow rate ofless than about 16 feet per second.
 12. A barrier seal system inaccordance with claim 3, wherein said oleophobic surface increases thewetting angle of the lubricating element and correspondingly decreasesthe required mass flow rate of said injected barrier gas to preventmigration of said lubricating element through said gap.
 13. A barrierseal system in accordance with claim 1, wherein said shaft furthercomprises a shaft sleeve.
 14. A barrier seal system in accordance withclaim 13, wherein at least a portion of said shaft sleeve disposedwithin said high-pressure region comprises an oleophobic surface;
 15. Abarrier seal system in accordance with claim 1, wherein said barrier gasis an inert gas.
 16. A barrier seal system in accordance with claim 15,wherein said inert gas is nitrogen.
 17. A barrier seal system inaccordance with claim 3, wherein the static contact angle of theoleophobic surface is greater than about 30 degrees.
 18. A dry gas sealassembly comprising: a dry gas seal; and a barrier seal assembly spacedapart from said dry gas seal comprising: a rotatable shaft disposedwithin an outer housing and axially extending from a high-pressureregion to a low-pressure region, wherein at least a portion of saidrotatable shaft disposed adjacent said low-pressure region comprises anoleophobic surface; a barrier seal that radially extends from said outerhousing to said rotatable shaft and defines a gap between the barrierseal and the rotatable shaft; a barrier gas injector for injecting abarrier gas into said gap to flow from said high-pressure region to saidlow-pressure region.